1. Field of the Invention
The present invention relates to a capillary type viscosimeter, and more particularly to a viscosimeter using a pressure difference between two points in a fine tube through which a fluid flows.
2. Description of the Prior Art
A continuous capillary viscosimeter is used for measuring viscosity of fluid continuously based on either a pressure difference between two points of a capillary through which the fluid supplied from a constant volume pump flows and an amount of the fluid flowing in the capillary under the Hagen Poseuille Law.
In the conventional viscosimeters of the above type, as shown in FIG. 1, fluid flow is taken in an inlet port 1 by a constant volume pump 3. All of the fluid flowing into a high pressure chamber 4 is taken in a capillary 5. The fluid flows to a chamber 6 and in turn an outlet 2. A differential pressure sensor 8 is connected to the chambers 4 and 6 through pipes 7.
In order to measure the viscosity using the Hagen Poseuille Law, it is desired that the fluid flowing the capillary 5 is a laminated flow. Therefore, the discharge of the constant volume pump 3 should not be large and it takes time to flow from the inlet 1 of the measuring fluid to the inlet of the capillary 5. Particularly, when a pressure diaphragm is provided for taking out the pressure difference, since the diameter of the diaphragm is 90 mm, a long measurement time is needed.
Besides, in order to measure the viscosity of fluid of low viscosity with a high accuracy, the capillary must be long. In case of using a straight capillary, the capillary must be very long and the viscosimeter becomes bulky. In order to avoid the inconvenience, it has been proposed to form the capillary in a spiral shape.
According to the conventional measurement of the viscosity, viscosity .mu. can be calculated by the equation (1). ##EQU1## wherein l: length of the capillary,
r: radius of the capillary PA1 .rho.: density of the fluid to be measured PA1 n: tube end correction factor of the capillary PA1 m: kinetic energy factor PA1 R is the radius of curvature of the capillary.
Normally, the viscosimeter is so designed that ##EQU2## so as to make the relation between .DELTA.P and .mu..
Then the equation (1) can be expressed as 0107 ##EQU3##
A problem of the conventional measurement of viscosity using the above equation (3) is in that the accuracy of the measurement becomes low in the low viscosity region due to the fact that the accuracy of the differential pressure sensor is affected by the valve of the measured pressure difference relative to the full scale value of the available measurement range. Thus, the accuracy becomes poor in the low viscosity region.
Since the accuracy of the differential pressure sensor directly affects the accuracy of the measurement result of the viscosity, it is difficult to obtain a measurement result whose accuracy is higher than the accuracy of the pressure difference sensor.